Continuous sulphate process



Ari] 1930- w. c. MUNRO 1,756,432

CONTINUOUS SULPHATE PROCESS Filed April 13. 1927 INVENTOR.

I BY

A TT ORNE Y.

to discard the spent lime sludge, the disposal Patented Apr. 29, 1930UNITED STATES PATENT-T OFFICE WILLIAM CAULDWELL MUNRO, OF MONTREAL,QUEBEC, CANADA, ASSIGNOR TO WILLIAM D. MOUNT, OF LYNCHBURG, VIRGINIACONTINUOUS SULPHATE PROCESS Application filed April 13, 1927, Serial No.183,368, and in Canada My invention relates to pulping processes for themanufacture of pulp from wood for the production of paper, andparticularly to the sulphate process including the kraft process, andthe method of preparing the cooking orpulping liquor used therein.

1 An object of my invention is to provide a continuous process formanufacturing the cooking liquor used in the sulphate or kraft processfor the pulping of wood, by a doubly cyclic process in which recovery ofthe ch emicals is practised in order to reduce the loss of materials andto keep the cost at a minimum. My process includes a cyclicreutilization of the soda compounds for the production of the pulpingliquor and also the cyclic reutilization of the causticizing lime,whereby no troublesome by-products are produced which must be disposedof, and whereby a large share of the cost of labor is avoided, and onlysuch amounts of soda compounds and lime must be supplied to the processas are necessitated by the minor losses inevitable in any process.

, My improved rocess includes a continuous clarification of the greenliquor by decantation, a continuous causticizing thereof, a continuousseparation of cooking liquor from the lime sludge, a continuousfiltering and Washing of the lime sludge, and a continuous ,burning ofthe sludge for the production of lime adapted to re-use in thecausticizer, as well as other continuous processes incidental to thesulphate process.

In the prior art of the pulping of paper by the sulphate process, thevaluable solids of the spent pulping liquors are customarily recoveredin the form of a crude black ash which is smelted and dissolved toproduce the green liquor, which, after causticizing and the separationof the lime sludge, becomes the white liquor for use in the digesters.To the extent that the soda compounds are reused, the former processesare partially cyclic and. continuous. It has, however, been customary ofwhich has been a source of much difficulty to the operation of thepulping plant, since the clear, sulphate, P

eentrated in the usual March 1, 1927.

the quantity is large and no commercial use existed for the contaminatedlime sludge.

By my invent-ion I prevent the troublesome accumulation of impuritiesin-the lime and reburn the spent lime from unit for the re-activationthereof, so that it may be re-used for further causticizing, therebyavoiding the need for the large quantity of fresh lime otherwisenecessary, and simultaneously avoiding the troublesome problem ofdisposing of the large quantity of calcium carbonate sludge.

My invention operates briefly as follows the causticizing The recoveredand reburned lime, together with a small make-up quantity of fresh lime,is charged into slaking tanks, with water and filtrate from the washingfilters, for the production of a slaked product which may, if desired,be in the form of a milk of lime./ I may use a plurality of slakingtanks, and while one tank is being charged, the slaked contents ofanother are pumped into a continuous causticizer, with a proper of thegreen liquor. A reaction is brought about therein under conditionsadvantageous to high efiiciency of reaction, by means of a current ofsteam and'air'. Suitable means are rovided for the regulation of thesupply both pf the green liquor and of the lime, whereby very accuratecontrol of the operation is obtainable.

The green liquor is subject to careful preparation before causticiv'ing.The spent pulping liquor from the digesters is desirably con- 7 way,reduced to substantial dryness in incinerators and smelted .in acontinuous smelter in the usual fashion. The smelted product is run tothe dissolving tanks in the usual way, and the-usual recovered greenliquoris thus obtained. I The a concentration of the liquor in thedissolving tanks is adjusted to a suitable strength and condition, suchthat the solid impurities will settle rapidly. The liquor isthen treatedby a continuous decantation for th emoval of the solid impurities,whereby a clear solution is obtained. This solution is desirably heldinstorage, so that a substantial supply is maintained at all times butit may if desired be sent direct to the caustieizer. From the quantity 7storage tank the clear liquor is then desirably sent to the continuouscausticizer as before described. After causticizing the sludge. may beremoved from the liquor. The inclusion of the continuous decanter actingupon the impure green liquor is an important part of the system ofmyinvention, since if the inipurities are allowed to enter the causticizerthey contaminate the lime sludge to such an extent as to preventsatisfactory reburning and reuse. The impurities separated from thegreen liquor by the decanter are desirably washed upon a suitablefilter, which may be a rotary filter, the operation of which may befacilitated by the addition of a suitable amount of sand or other filteraidto prevent the passage through of colloidal material. The relativequantity of impurities so obtained is small, though of sufficientquantity then may be pumped to a to seriously contaminate the limesludge, and the impurities being of no economical value, may bediscarded and disposed of without difficulty. The wash water from thefilters contains a substantial amount of dissolved soda compounds ofvalue, which may be returned to a suitable point in the cycle, such asthe dissolving tank or the green liquor storage tank.

The causticized liquor may be separated from the lime sludge by asuitable means such as a continuous decanter/ The white liquor suitablestorage tank for ultimate delivery to the digesters. The sludge may betransferred to suitable sludge filters for the recovery of further sodasalts and to prepare the lime for the lime kiln. In the kiln the sludgeis reburned further causticizing.

By this means I produce a doubly cyclic system for the pulping ofwood,in which a recovery both of the pulping agent and the causticizing agentoccurs. By this means I avoid the loss of substantial values ofthe lime,and avoid the necessity for the purchase of large quantities of freshcausticizing lime, as well as avoiding the necessity for disposal oflarge quantities of spent materials.' The system of my invention hasother important advantages, such as compactness of installation, lowerlabor costs in the operation thereof, lower repair charges, much lowerchemical losses, lower steam consumption, greater uniformity of product,thesaving of lime previously mentioned, and a substantial, valuable andimportant increase in efficiency.

Other objects and structural details of my invention will be apparentfrom the follow-- in g description the accompanying drawing, wherein Thesingle figure a diagrammatic representation of the apparatus of myinvention, showing the elements, the connections therehetween, and thedirecton of travel and sewhen read in connection with to lime which maybe re-slaked and used for;

quence of operations incidental to the opera- '-tion of my process.

In the drawing, a group of digesters 1, 2,

3 and 4 are provided, which may be of any pulping of wood suitable formadapted to the liquor. The dibythe kraft, or sulphate.

gesters are supplied with fresh, white, cook- I ing liquor from astorage tank 5, and after the completion of the reaction in the usualthen transferred to an incinerator unit 8 in which the water is removedto reduce the chemical compounds nearly to dryness to produce a blackash. usual smelter unit 9, from which the molten smelt is transferred toa dissolving tank 10 and taken up in a suitable quantity of water toform what is known in the art as green liquor.

an aqueous solution of sodium carbonate, sodium sulphide, certainquantities of solid impurities such as silica, ferrous sulphides and thelike. The foregoing steps are a sequence which is old in the art and incommon use.

It is in the succeeding steps in which the features of my invention lie,including means for the prevention of contamination of the ca usticizinglime and the cyclic reutihzation thereof.

In the prior art it has been customary to transfer the green liquordirectly from the dissolving tanks. to the causticizing units. By myinvention, however, thegreen liquor is transferred from the dissolvingtank 10 to a continuous clarifier or decanter for the removal of theinsoluble and suspended imand sodium hydroxide, with This is dischargedto a This product consists essentially of v In the cold, or if allowedto cool, the character of the green liquor solution and suspendedimpurities is such that they settle with extreme slowness. It has beenconsidered impossible in the past to separate continuously these.impurities directly from the green liquor. by means of the methods andequipment so far. utilized for attempting the continuous separation. Ifind, however, that if provision is made to continuously separate theimpurities from the green liquor before the green liquor loses theconsiderable heat it possesses on leaving the dissolving tanks,

' obtained from the red'hot smelt, a change occurs in the character ofthe constituents in their relation to the hot green liquor which causesthe impurities to settle much more rapidly, and I accordingly makeprovision and transferred to a filter 14 through the pipe 15. In thefilter the remaining green liquor is separated from the impurities, andreturned by a suitable pipe to the storage tank 13. In some instancesdifiiculty may be encountered in obtaining a clear filtrate, because ofthe tendency of theimpurities to assume a colloidal condition. lVhenthis occurs a small amount of sand or other filter aid may be added tothe subflow from the decanter before it reaches the filter. The solidsin the sludge are small in quantity and accordingly they. are discarded,after separation in the filter 14, without difliculty. Being small inquantity, they carry with them a negligible amount of the valuablepulping agent and they may accordingly be discarded directly from thefilter, or they may, if desired, be washed by suitable treatment for therecovery of such soda values as occur in them, in which case the washwater may be returned to the system at a convenient point.

The clarified green liquor is drawn from the tank 13 and transferredcontinuously into a continuous causticizer 17, and mixed with milk oflime from the slaking tanks 18.

The supply of lime is desirably provided by the inclusion of one or morelime kilns 19" n the plant, which may desirably be of the" rotarypattern, since this is best adapted to the r'eburning of the recoveredsludge. The kilns 19 are arranged to discharge the burned l me upon aconveyor 20, to which the fresh hmerequired for the make-up of lossesmay be added. The conveyor transfers thelime into the slaking tanks18'into which suitable slaking fluid, which may be clear water, or

may be wash water, is admitted through suitable pipes 21. The limeisdesirably held in suitablebaskets 22 by'means of which it is broughtinto contact with the contained water so that the lime will be mayslaked and transferred to the contents of the tank. The lime is thusconverted to the form of milk of lime and fully slaked. The tanks'areemptied by a suitable'variable speed pump 23 which is adapted to adjustthe flow of fluid in accordance with the flow of green liquor up thesmall losses in the cycle, such as those from the tank 13, into thecausticizer 17.

The causticizer 17 may be of" any convenient pattern, but-is desirablyone in which vigorous stirring occurs from the presence of hot air andsteam, whereby a thorough causticization in a minimum time .and with amaximum eificiency occurs. It may desir- --ly disclosed to the art.

ties, and" by suitable ably be a form of tower causticizer previous-From the causticizer the liquor and sludge are then transferredcontinuously to the sludge decanters 24 in which the lime sludge settlesto the bottom and the clear liquor rises to the top. The .underflow,containing the lime sludge and a small amount of causticized liquor,passes through the pipe 25 to the sludge storage tank 26. The clearoverflow is returned through pipes 27 to the storage tank for the whiteliquor from which it is supplied to the digesters 1,2, 3 and 4, therebycompleting the cycle for the pulping agent.

The sludge is passed from the storage tank 26 to filters 28 in which thegreater part of the liquor is removed. The removed liquor is returned tothe slaking tanks, at a convenient point to prevent loss of itscontained values. The lime sludge separated by the filters is desirablywashed to remove other values and the wash water likewise may bereturned to the slaking-tanks through pipe 21. The washed lime istransferred from the filters 28 to the lime kilns 19 and reburnedtherein, thereby also completing its cycle.

My invention thus provides a doubly cyclic process for the kraft orsulphate pulping liquor and the recovery and reutilization of thecausticizing lime. In my system I provide means whereby the green liquoris treated to agglomerate and settle the impurities, and remove themfrom the solution, thereby preventing a contamination of thecausticizing lime. As previously described, I prepare the ,green liquorin such a way that the solid impurities settle easily, and remove themby suitable means before further processing of the. liquor. I thenconvey the clear green liquor 'to the causticizing device, in which thesoda carbonate in the liquor is converted to hydroxide, and the limechanged to calcium carbonate. The spent limeZis, however, notcontaminated by impurities from the black ash-and the smelter, and afterrecovery in the form of sludge from the white liquor, it issubstantially free from troublesome impuritreatment"-during"thecausticizing operation, it'is caused to assume a form in which it isreadily reburned in the customary rotary kiln.

By this system of my invention, the soda compounds in the pulping liquorare cyclically re-used in the fashion common to the art, whereby onlysuch supplies of new pulplng agent are required as must be added to makewhich occur in the handling of any cyclic process. By this means aminimum of new lime is required, and a. minimum of waste materials mustbe discarded, thereby producing a very substantial saving in the cost ofmaterials for the stantial simplification of the operation of canter,separating solids from eration of the sulphate prising digesters for theproduction of a the plant.

The previous description discloses but a single method by which theprocess of my invention may be applied. It is, however, capable ofvarious modifications therefrom and the adaption of other elementstherein, without departing from the spirit thereof, and it is desiredtherefore that only such limitations shall be imposed thereon as arerequired by the prior art or indicatedby the a ppended claims. 4

I claim as my invention:

1. In a continuous process for preparing the pulping liquor for thesulphate process the steps of continuously supplying a quantity of greenliquor to a continuous decanter, separating solids from the liquortherein, in the form of sludge, filtering the sludge, returning thefiltrate to the process, discarding the solids, transferring thedecanted liquor continuously to storage and continuously withdrawing thestored liquor for causticization.

2. In a continuous process for preparing the pulping liquor for thesulphate process the steps of continuously supplying a quantity of hotgreen liquor to a continuous dcthe liquor therein, in the form ofsludge, filtering the sludge, returning the filtrate to the process,discarding the solids, transferring the decanted liquor continuously tostorage and continuously withdrawing the stored liquor forcausticization.

3. An apparatus for the doubly cyclic oppulping process, comreactionbetween a solution and wood, and the production of pulp and spentliquor, means for the recovery of the soda compounds in the spentliquor, comprising incmerator, smelter, and dissolver, means for theseparation of solid impurities therefrom comprlsing a continuousdecanter adapted to theoperation upon said dissolved soda compounds atan elevated temperature, means for the causticization of said sodacompounds comprising a causticizer positioned subse quent to saiddecanter in the cycle, means for the separation of spent lime from thesolution, and means comprising a filter and a kiln for the reactivationof said lime.

4. Apparatus for the operation of the sulphate pulping processcomprising means for the production of pulp and spent liquor,

means for the utilization and recovery of the soda compounds in thespentliquor, means for the recausticization thereof, comprising 111 )inrocess and a sub- P l a P a causticizing tower, and means fortheseparatlon of impurities, comprising a continuous decanter positionedbetween said recovery the alkali materials from said liquor in solid 7impure condition, the dissolving thereof to form the green liquor, theseparating of solid impurities therefrom by tion, the discarding of saidimpurities, the causticizing of the decanted liquor, the separating ofsludge therefrom by decantation and the reuse of the clear decantedliquor for digesting of wood. a

7. A process for producing sulphate pulp which comprises the digestingof wood by means of the alkali liquor, the pulp and spent liquor, therecovering of the alkali materials from said liquor in solid impurecondition, the dissolving thereof to form the green liquor, theseparating of solid impurities therefrom by continuous decantation, thediscarding of said impurities, the causticizing of the decanted liquor,the separating of sludge therefrom by decantation, the recovering oflime from said sludge the separating of.

continuous decantaby calcination thereof, the reusing of said lime forcausticizing, and the reuse of the clear decanted liquor for of wood.

8. A continuous cyclic process r producing sulphate pulp which comprisesthe digesting of wood by means of the alkali liquor,

the separating of the pulp and spent liquor,

the recovering of the alkali materials from said liquor in solid impurecondition, the dissolving thereof to form the green liquor, theseparating of solid impurities therefrom by continuous decantation, theof said impurities, the'continuous causticlzlng of the decanted liquor,the separating of sludge therefrom by continuous decantation and thereuse of the clear decanted liquor for digesting of wood.

9. A continuous cyclic process for producing sulphate pulp whichcomprises the digesting of wood by means of the alkali liquor, theseparating of the pulp and spent liquor, the recovering of the alkalirraterials from said liquor in solid'impure condition, the dissolvingthereof to form the green liquor, the separating of solid impuritiestherefrom by continuous decantation, said impurities, the continuouscausticizing the discarding of of the decanted liquor, the separating ofsludge therefrom by continuous decantation, the recovering of lime .fromsaid sludge by calcination thereof, the reuse of said lime forcontinuous causticizing, and the reuse of the clear decanted liquor ordigesting of wood.

In witness whereof, I hereunto subscribe my signature.

WILLIAM CAULDWELL MUNRO.

